This invention relates to a method and device for reducing noise adjacent the source thereof in air distribution systems. More particularly the apparatus comprises one or more sound attenuating modules which may be located near a noise source such as noisy machinery or added to conventional air distribution ducts. The method comprises the steps of constructing and employing the said device in a unique fashion to reduce noise.
The background and known prior art is best summarized in a book by Leo L. Beranek, entitled "NOISE REDUCTION" and published in 1960 by McGraw-Hill Book Company, Inc.
Modern air conditioning systems generate objectionable noise in a variety of ways. The most significant source of such noise emanates from the system fan which can produce a relatively low frequency hum as it forces air into or out of the associated ventilation system. Similarly, air conditioning apparatus such as cooling towers and compressors likewise can contribute to noise production through mechanical vibration. Turbulence generating discontinuities within distribution ducts, caused, for example, by right angle bends, mixing boxes, heat exchangers and other apparatus, can produce objectionable noises as air rushes through the system. Often the areas or rooms receiving conditioned air contribute to the noise problem by introducing sounds (from conversation or typing, for example) into the distribution network via the terminal duct in the particular room.
Prior art noise reduction systems are known to employ glass or mineral wool fiber layers interiorly of ventilation ducts to absorb sounds from the air rushing therethrough. Sound energy is dissipated as acoustically excited vibrating air particles rub against relatively stationary exposed glass fiber or mineral wool strands. Fibrous material is normally placed on the interior sides of the ducts and, where desirable, at the center portions thereof. In one common form of prior art noise reducer, glass fiber material or mineral wool is housed within a perforated metal enclosure so that acoustically excited air particles may pass therethrough to contact the glass fiber as the relatively steadily flowing air passes along the surface. Each housing is located interiorly of a specially designed noise reduction duct which usually must be installed in series with an existing duct to reduce noise. The sound absorbing media may be positioned either against the side walls of the duct or spaced between the sidewalls, or both.
The above-mentioned prior art noise reduction devices are disadvantageous in several respects. Installation of these devices often necessitates special sheet metal fittings to adapt the housing to the main duct system, resulting in the usual delay and expense associated therewith. Since the rough glass fiber noise dampening material contacts the air running through the duct system, air friction is increased. Consequently a larger (and usually noisier) fan must be employed to maintain a given air flow through such a system. As a corollary thereto, the exposed glass fiber will tend to filter or absorb particles or vapors carried by the air passing through it or by it. By way of example, prior art systems in hospitals, laboratories and the like tend to collect germs or fungus. Similarly, when such systems are used in machine shops, they tend to accumulate a variety of petroleum vapors. Besides contributing to environmental hazards in the manner just described, various particles and foreign material absorbed by the glass fiber inevitably decrease noise reduction capabilities by inhibiting the passage of acoustically excited vibrating air particles. In high velocity air distribution systems, friction between the glass fiber sound absorbing material and air rushing thereby can lead to deformation of the glass fiber panels. Furthermore, where perforated metallic housings are used to reduce glass fiber deformation, additional high-frequency noise can actually be generated as air rushing by the perforations creates a variety of "whistling" or "rushing" sounds. Also, the perforations eventually tend to clog as airborne material is deposited therein.
The present invention includes a unique method and device for reducing noise which obviates the aforementioned problems. The device comprises a noise reduction module which may be located adjacent the noise source (factory machines and the like) or easily inserted interiorly of an existing air duct by sliding it therein and fastening it to the duct by sheet metal screws. The module comprises a plurality of compliant glass fiber sheets for absorbing and dissipated unwanted sound energy, but unlike prior art devices the glass fiber does not contact the air passing through the duct. Instead, a plurality of sandwiched glass fiber sheets are completely and sealable housed within a thin metallic airtight enclosure. Each internal sheet is separated from the adjacent sheets(s) within the enclosure by a similar impermeable membrane. When located in a duct, a generally triangular or wedge shaped member is provided at each end of the module to reduce air turbulence as the conditioned air passes around the module.
In general the shape of the module may be modified and its dimensions can be varied, where necessary to accomodate duct work of varying sizes and shapes. Thus the normally rectangular module can be adapted for use with round, oval or other irregularly shaped ducts. Also, modules can be located near right angle turns or bends and modified with turning vane end members for accordingly reducing objectionable noise normally produced by the duct angles.
The compliant glass fiber sheets in the module alternately contract and expand in response to noise caused by compression and rarefaction of the air passing through the duct or around noisy equipment. Sound energy is dissipated, for example, in the form of internally generated heat as the glass fiber sheets are alternately compressed and expanded. Since the walls (foil membrane) of the module are smooth and lightweight and further since the moving air through a duct does not contact the glass fiber, frictional air stream losses are minimized to thereby provide a noise reduction system of improved efficiency. Importantly, the acoustic impedance of the module at the frequency to be attenuated is minimized by presenting an acoustic path equal in length to approximately one quarter wavelength of the frequency to be attenuated. Thus, unlike known prior art sound absorbers, acoustical "matching" is employed by the present invention in a manner acoustically equivalent to principles of electrical transmission line theory.
The media is "matched" to the air outside of the device when the reactances contributed by the mass and compliance cancel to present a net reactance of zero to the impinging sound wave, and when the various resistances in the media combine and are transformed by the reactances to present a net resistance equal to the characteristic resistance of air.
The subject method comprises, in part, the steps of selecting at least one preferably glass fiber layer having a total thickness approximately equal to one quarter wavelength of the sound frequency to be attenuated, sealing the layer within the airtight membrane having at least one flexible wall portion adjacent the layer, thereby forming a noise attenuating module, and locating the module adjacent a source of noise or installing same interiorly of the air distribution ductwork. Where necessary, space can be conserved by the additional step of increasing the density (within limits) of the glass fiber layer to reduce the velocity of sound wave therethrough, thereby reducing the equivalent acoustical quarter wavelength at the frequency of interest. In any event, the aforementioned method does not contemplate laborious sheet metal modifications of or additions to existing air distributing apparatus.
Thus, a primary object of this invention is to provide a unique noise reduction method and device which has particular utility with air distribution systems.
A further object of this invention is to provide a noise attenuating method and device of the character described which can easily and quickly be utilized in conjunction with existing duct work without complex sheet metal alterations.
It is also an object of this invention to provide a compact and efficient noise attenuation module for air distribution ducts. It is an important feature of the invention that more than one module may be conveniently installed in a spaced relationship, if needed. Accordingly, manufacturing costs may be reduced since the total noise reducer can comprise one or more modules at a single location, obviating the need for custom designs.
Another object of this invention is to provide a noise reduction device of the character described which, upon installation, presents negligible airstream resistance.
A further object is to provide a unique device for reducing noise which may be operatively positioned adjacent to a noise source in an easy and convenient manner. It is a feature of the invention that the device may be effectively hung or located near noisy machinery and the like to reduce the level of noise that an operator would normally experience.
A related object of this invention is to provide a noise reduction device of the character described above in which glass fiber dampening material is adapted to reduce noise without directly contacting the air passing through the vent.
Another object of this invention is to provide a noise reduction module having optimal acoustic input impedance at the noise frequencies of interest and to further provide a method for reducing heretofore excessively high acoustical impedance at low frequencies.
Yet another object of this invention is to provide a noise reduction module of the character described which will attenuate a relatively wide range of noise frequencies. It is a feature of the invention that multi-stage "matching" networks can be formed by using properly selected different density layers within the module. Further, glass fiber layers of different densities can be employed in adjacent modules to simultaneously attenuate different noise frequencies.
A still further object is to provide a noise reduction module which can be varied in size and shape to suit a variety of installations. It is a feature of the invention that the module may be in the form of a cowling located around a fan unit, a rectangular device of the type described, or arcuately shaped for air turning purposes as well as noise reduction.
Yet another object of this invention is to provide a noise reducing module having the attributes and features mentioned aove which is rugged, dependable and long lived.
Other and further objects of this invention will occur in the course of the following detailed description.